The effect of hydrostatic pressure treatment on the induction of mitogynogenesis in the eggs of Japanese flounder Paralichthys olivaceus (Temminck et Schlegel) by using heterospecific sperm were studied. Before treatment, the eggs were at metaphase of the first mitosis. The spindle was disassembled by the treatment and then resembled in its pretreatment position, and the chromosomes were rearranged, i.e., the first mitosis was not blocked. During the second mitotic cycle, only a monopolar spindle was assembled in each blastomere and the chromosomes doubled, but cell cleavage was blocked. In the third cycle, mitosis proceeded normally with a bipolar spindle in each blastomere. Flow cytometric analysis of ploidy demonstrated that mitogynogenetic larvae were all diploid. The ultraviolet light-irradiated sperm of the red sea bream (Pagrus major) was condensed, formed a dense chromatin body, and randomly entered one blastomere.